It is well known to form a vent hole in an airbag to allow gas to be expelled from the inside of the airbag through the vent hole when a vehicle occupant plunges into the inflated airbag, whereby the vehicle occupant is softly received and restrained by the airbag.
JP2000-16228A discloses an airbag in which a vent hole is covered by a lid member until the inner gas pressure of the airbag reaches a predetermined value, and the lid member is released to open the vent hole when the inner gas pressure reaches the predetermined value.
In the aforementioned publication, description is made as regard to a driver-side airbag of a vehicle. The airbag disclosed in the aforementioned publication is made by sewing the peripheries of two circular sheet members (panels) together to form one chamber as a whole. The vent hole is formed in the sheet members on a side opposite to the vehicle occupant side of the airbag. The lid member is arranged to cover the vent hole.
In the airbag disclosed in the aforementioned publication, the vent hole is covered by the lid member until the inner gas pressure of the airbag reaches the predetermined value so that the outflow of gas through the vent hole is restricted when the airbag is inflated, thereby rapidly making the inner pressure of the airbag high and thus rapidly inflating the airbag.
As the inner gas pressure of the airbag exceeds the predetermined value, the lid member is released so as to open the vent hole. Therefore, gas flows out from the inside of the airbag through the vent hole when the vehicle occupant plunges into the inflated airbag, whereby the vehicle occupant is softly received and restrained by the airbag
Patent Document 1: JP2000-16228A
In the publication JP2000-16228A, the vent hole may be opened even in a stage before the occupant plunges into the airbag if the inner pressure of the airbag is increased to the predetermined value or more so that gas is unfortunately flowed out through the vent hole.